1. Field of the Invention
The present invention relates to a method, for refining a chromium-containing molten steel by decarburization, which can improve the decarburization rate, prevent the oxidation of [Cr] contained in the molten metal and enable the molten steel to be decarburized with a high efficiency.
2. Description of the Prior Art
A chromium-containing steel, such as stainless steel, has hitherto been refined by a combined-blown process wherein an oxygen gas or an oxygen-containing gas (hereinafter referred to simply as "oxygen") or a gas mixture of oxygen with an inert gas is blown into a molten steel through a tuyere provided at the bottom of a furnace (a bottom-blown tuyere) and through a lance provided above the surface of the molten steel (a top-blown lance).
Japanese Unexamined Patent Publication (Kokai) Nos. 55-158213 and 61-266516 describe a method for efficiently decarburizing a molten steel while preventing the oxidation of chromium contained in the molten steel, wherein a secondary combustion reaction, which converts CO generated on the surface of a molten steel bath to CO.sub.2, is positively allowed to proceed to raise the temperature of the molten steel by taking advantage of the heat of reaction, thereby preventing the formation of chromium oxide to reduce the Si content which is necessary for the reduction of chromium oxide in slag (unit requirenet of Si for reduction).
More specifically, Japanese Unexamined Patent Publication (Kokai) No. 55-158213 describes that oxygen or an inert gas is blown into below the surface of a molten steel bath to decarburize the molten steel while feeding, from above the molten steel, oxygen in an amount of at least 0.2 times the amount of oxygen fed into below the surface of the molten steel, allowing a secondary combustion reaction, which converts CO generated on the surface of a molten steel bath to CO.sub.2, to positively raise the temperature of the molten steel by taking advantage of the heat of reaction, thereby preventing the formation of chromium oxide to reduce the amount of Si which is necessary for the reduction of chromium oxide in slag.
Japanese Patent Unexamined Patent Publication (Kokai) No. 61-266516 proposes the following formula representing the relationship between the proportion (P) of the top-blown oxygen, which reacts with the molten steel, in the top-blown oxygen, the lance height (L), and the velocity (V) of the top-blown oxygen blown through the lance and describes that a desired proportion of top-blown oxygen, which reacts with the molten steel, can be attained by blowing the top-blown oxygen at a velocity in the range of from 150 ft/sec (45.7 m/sec) to the velocity of sound: EQU P=K-1629(L/V)
wherein
P: the desired proportion of top-blown oxygen which reacts with the molten metal (%), PA1 K: a constant in the range of from 56 to 72, PA1 L: the height of the opening in the lance above the surface of the molten steel (ft), and PA1 V: the flow rate of oxygen blown through the lance (ft/sec).
Considering the fact that the secondary combustion reaction caused by top-blown oxygen is generally recognized as occurring at a flow rate of oxygen in a region of lower than the velocity of sound (a free jet region), it can be said that the method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-266516 is intended to allow the secondary combustion reaction caused by the top-blown oxygen to positively proceed.
In all the above prior art, in order to improve the decarburization efficiency in refining a chromium-containing molten steel, the secondary combustion reaction caused by top-blown oxygen is positively allowed to proceed, and the temperature of the molten steel is raised by taking advantage of heat generated by the secondary combustion reaction. In order to raise the temperature of the molten steel by taking advantage of heat generated by the secondary combustion, it is necessary to ensure a supply of oxygen in an amount necessary for the secondary combustion. When the combined-blown process and the bottom-blown process are compared with each other under the same oxygen feed rate conditions, the necessary decarburization refining time for the combined-blown process is unfavorably longer than that for the bottom-blown process, because the amount of oxygen necessary for the secondary combustion for the combined blown process is larger than that for the bottom-blown process, causing the amount of oxygen, which reacts directly with carbon contained in the molten steel, to be reduced.